Earlier this month, women in the U.S. at risk of developing breast cancer received a limited reason for hope: A drug called tamoxifen was found effective at preventing breast cancer, but it is associated with increased rates of uterine cancer. Now comes a potentially bigger breakthrough. As-yet-unreleased results of clinical trials of a drug called Evista indicate that it, too, is effective at preventing breast cancer-without any risk of uterine cancer.